With rapid growth in customer demand for ever-increasing bandwidth to support transmission of interactive voice, video and data, the telecommunications industry has outgrown the capability of copper wire. The hunger for a supply of information at first moved slowly from the 56-kbps rate offered by dial-up modems to the greater bandwidth offered by DSL. However, access to broadband transmissions at the 640-kbps capacity of DSL only served to whet the appetites of consumers of data who have come to demand even higher rate feeds as more applications are developed to access and use more and more data, such as HDTV and video on demand. To extend their reach beyond the 10-km limitation imposed by DSL over copper wire, telecommunications and cable providers have been forced to employ optical fibers and extend them ever closer to more of their customers.
Optical fibers are capable of carrying virtually noise-free signals that may be extended for very long distances without the need for an amplifying repeater. This has allowed fiber optic networks to move closer to the home and office. Often these greater distances exceed the length of optical fiber cables that can be manufactured, requiring the inclusion of splice points along the route. In other cases a bundle of fibers may need to be tapped at a drop point to provide access to customers along the route. Fiber to the Home, Neighborhood and Office (FTTH, FTTN, and FTTO) presents new challenges in fiber management for routing, splitting, and combining signals as these so called Deep-Fiber (DF) applications are deployed.
Outside-plant installations of optical fibers typically package twelve fibers in a loose-tube cable. In high-density service areas, loose-tube cables containing 36 or as many as 200 fibers may be used, in which case the cable is generally made up of multiple smaller bundles of fibers packaged in color-coded plastic buffer tubes. This modular buffer tube design permits easy drop-off of groups of fibers at intermediate points along a route without interfering with other protected intact buffer tubes being routed to other locations. The loose-tube cable configuration is amenable to aerial, duct and direct-burial applications.
To make splices and feed drop points, the optical fibers within a buffer tube must be exposed by removal of their protective coverings. As each individual fiber is spliced to another fiber, by fusion or other mechanical techniques, to extend the reach of the transmission, the resulting splice must be physically supported and protected from the elements before being placed into service. The splicing operation is often carried out in a transportable clean-room which can be relocated as necessary to access the fiber optic cables on location. Since the cables will either be suspended from utility poles or buried underground, there must be sufficient excess cable to accommodate movement of the splices to the clean work environment. Furthermore, there must be sufficient excess fiber length relative to the length of the buffer tube, typically from one to four meters of slack, to allow the technician to comfortably test and organize the individual fibers.
A variety of splice trays have been developed to provide support for the splices and specialized housings have been produced to protect the splice trays from the environment. Most of these trays also accommodate sufficient storage of the excess slack fiber that is no longer protected by a buffer tube. One popular “Fiber Optic Splice Closure” (FOSC) has been described by Mullaney, et al. in U.S. Pat. No. 5,323,480. Assigned to Raychem Corp., this FOSC has been developed into a series of systems of splice trays with associated outer protective housings. Many other parties have developed similar assemblies some of which may be interchangeable. U.S. Pat. No. 5,074,635 issued to Justice, et al. and assigned to the 3M Company, disclosed what has become a popular splice tray.
Splice trays are generally designed with an attempt to offer some organization for individual splices. This attempt is challenged and limited in many regards. Though a splice tray must permit the supportive mounting and protection of individual splices and storage of the slack optical fibers in a relatively neat configuration, space is limited. Storage of a sufficient amount of slack to enable splicing is most often achieved by forming the optical fibers into a series of loops. However, a minimum bend radius must be carefully observed when handling and storing optical fibers. Bending a fiber more sharply than a certain radius will, at first, result in increased attenuation of the optical signal. At some tighter radius the fiber will break. The minimum bend radius for a given set of fibers establishes some minimum dimensions for splice trays and their enclosures.
Another consideration in the organization of splices within a tray, or a system of trays, is that the loops of slack fibers frequently become permanently tangled with those of neighboring fibers. This occurs simply in the normal course of splicing and routing of the slack for storage. Occasionally a splice will need to be accessed at a later date in order to be remade or repaired to achieve a splice of sufficient quality for support of a proper optical signal. The Fiber Optic Splice Organizer described in U.S. Pat. No. 5,278,933 to Hunsinger, et al., issued Jan. 11, 1994, is one attempt for securing optical fiber splices and slack which permits separation of the spliced fibers to facilitate remaking of a splice. U.S. Pat. No. 6,507,691, issued to Hunsinger, et al., is another one in a series of patents owned by Tyco/Raychem which includes the above-mentioned Mullaney ('480) patent. Taken together these patents show the use of a plurality of splice trays hinged together at a common end and mounted within a single housing.
A stack of splice trays that are so configured offers some capability for segregating buffer tubes from one tray to another, but provides no assistance for organizing fibers once removed from the buffer tube. Such a system of stacked trays limits access to one tray at a time. In U.S. Pat. No. 4,913,522, Nolf, et al. make a point of mentioning that with their (Raychem) tray design “hinging allows chosen trays to be exposed for installation of the splice or for repair etc.” This is an advantage in that only one tray at a time is exposed to potential damage while the fibers stored in all other trays remain protected. A disadvantage of such a system is that access to the fibers contained by any particular tray is restricted by the extent to which other trays within the stack may be folded out of the way or the working tray may be removed from the stack, without over-bending any of the fibers.
With perhaps as many as 300 users being supplied by three optical fibers, a single twelve-strand buffer tube in a fiber cable can provide networking to many housing subdivisions. This high-density is convenient for new installations that have not yet gone “live” and provides efficient use of currently deployed fibers. Once an optical network has been activated, however, handling of a single delicate fiber that is feeding a large number of subscribers carries a significant exposure to liability which often corresponds to very large insurance premiums paid by those technicians who work in trays containing active fibers. Furthermore, service providers are at risk of dissatisfying customer with unscheduled outages if fibers are broken. This risk suggests that active (Light, live, or hot) fibers should be segregated and provided with greater protection than those that are inactive (Dark, dead, or cold) while storing the latter for future expansion of the network.
To be useful a splice tray must provide support for splices that have already been prepared and for access to those splices for repair or for activation as the network is extended. The tray must also provide adequate storage for slack fiber associated with the splices without bending any individual fiber more tightly than its minimal bend radius.
It would be an advancement to the state-of-the-art to provide a system of splice trays that would allow for categorization of fibers removed from a single buffer tube as well as between buffer tubes. An additional advancement would be provision for management of Dark, inactive fibers for future expansion without disturbance to Light, active ones. The ability to simultaneously access multiple trays within an enclosure, without folding other trays out of the way as has been required by previous systems, would be a further advancement.